As has been described in the aforementioned copending application and the corresponding Europatent publication, it is possible to produce upon a substrate, e.g. a glass pane, plate or foil, a solid layer which is translucent and may even be transparent, by the drying of an expandable material namely a hydrated alkali metal silicate by drying a water solution thereof on a substrate.
The solution can also include auxiliary materials and in general the drying is carried out by heating the layer in a drying chamber with a drying atmosphere critically chosen to minimize bubble and microbubble formation which might affect the light transmissivity characteristics.
Circulating water vapor and helium can form this atmosphere and the drying atmosphere is held at about 100.degree. C.
The fire-resistant layer retains its effect when the layer is separated from the substrate or is integrated into a plate or structure, e.g. a light-permeable part of a plate structure, by being sandwiched between two glass panes, and whether the layer is in the form of a plate or a foil.
In general the compositions of the layers described in the aforementioned copending application and the uses thereof will be applicable here as well, and, as a consequence, the aforementioned copending application is hereby incorporated in its entirety by reference.
The substrate can, as noted, be a glass pane or a foil which remains bonded to the layer or from which the layer is removed. The layer is therefore either left on the substrate or otherwise incorporated in a fire-resistant wall or a portion of a fire-resistant wall of a structure. The auxiliary materials which can be included in the solution and may find their way into the layer are those named in the aforementioned copending application such as urea, di- or polyhydric alcohols, saccharides, sodium phosphate, sodium aluminate, aluminum phosphate, borax and boric acid.
As noted previously, the aforementioned copending application describes the fact that the choice of the drying atmosphere is critical to the properties of the product which is to result. In that case, as in the Europatent publication, the drying atmosphere must contain at least 30 volume percent oxygen and a maximum of 15% by volume in total of one or more of the gases argon, nitrogen and sulfur hexafluoride, the balance being water vapor and/or helium.
Bubbles and microbubbles can thus be avoided in the layer which is produced so that the layer has excellent light transmissivity properties and the aging characteristics of the layer are good under most conditions and at least until the layer is altered by exposure to fire to the point that its fire-protective function is lost.
However, the aging resistance, especially under the effect of light and solar radiation, can be improved further.